The manufacture of white portland cement generally consists of carefully selecting raw materials that are otherwise conventional portland cement ingredients but very low in colorants such as iron or chromium compounds, blending said raw materials, burning them at high temperatures in a rotary kiln, and rapidly quenching the resulting clinker in the absence of air, or cooling in the presence of a reducing material such as oil, coke, natural gas. The steps of quenching and reducing may be combined.
The coloring action of the metal cations such as Fe.sup.3.sup.+ or Cr.sup.3.sup.+ depend upon the valence state of the cation and its coordination number in the crystals of the cement phases. High temperature (over about 1100.degree.C) causes an increase in the coordination number of the colorant ions in the crystal lattices. The higher the temperature or the higher the concentration of reductant, such as CO or H.sub.2, the more ions are aligned in the octahedral configuration. In the octahedral configuration, the polarization of the colorant ion is weakened thus decreasing the amount of adsorbed light rays and therefore increasing the amount of diffused reflected light. Thus, to the eye, the crystals, and the cement, appear whiter. The transition from a lower to a higher coordination number is enhanced by the reduction of the colorant oxide. If the material is allowed to cool slowly from the kiln temperature to a temperature between 2200.degree.C to 1100.degree.C or less in the presence of oxygen, the colorant will re-oxidize to some extent and re-establish itself at a lower coordination number in the crystal thus allowing more light adsorption by the colorant which will manifest itself as a more perceptible color by the eye.
If the cooling of the material can be accomplished rapidly so as to "freeze" the atoms in the lattice at the higher coordination number, by limiting the atomic amplitude in the crystal, then the colorant ion will remain at a stable valence and coordination number. Thus it is found that rapidly cooling or "quenching" the clinker material from at least 1100.degree.C (2000.degree.F) to below 535.degree.C (1000.degree.F) and preferably from about 1500.degree.C (2730.degree.F) to below 535.degree.C (1000.degree.F) in the presence of a reducing fluid or in the absence of oxygen will produce a clinker capable of being ground to a white cement. If the clinker is cooled slowly in the presence of oxygen or is cooled rapidly to a temperature above 1000.degree.F and then cooled slowly to below 1000.degree.F in the presence of oxygen, the clinker will be darker than desired. Therefore, it is advantageous to quench the clinker from as high as possible to below 535.degree.C as rapidly as possible.
In accordance with the present invention, it has been observed that certain sizes of white clinker made by the above process are whiter than others. High reflectivity white portland cement is a more marketable product than darker shades of white portland cement since its "whiteness" is the characteristic sought.
Heilman, in British Pat. No. 857,105 teaches that white clinker in sizes greater than one inch should be screened out of the kiln discharge mass after quenching and, being darker, should be crushed and returned to the feed end of the kiln. However, Heilman has neither determined an optimum size of clinker nor the effect of controlling the process to achieve or utilize an optimum size related to whiteness.